A laser cut metal mask is a metal plate which has small holes (or slits) in a prescribed patterned image which are formed by melting and boring holes in a portion of a metal plate by irradiating the metal plate with a laser beam. It is primarily used for screen printing of solder paste (solder cream) on a circuit board such as a printed wiring board with a squeegee, but it is also used in other applications such as printing of electrically conductive ink. Plates made of stainless steel, titanium, titanium alloys, aluminum, aluminum alloys, nickel, and the like are used as metal plates. In the past, holes were formed in a metal mask by etching, but with the spread of laser processing machines, laser cut metal masks in which it is possible to form holes with higher precision have come to be much used.
Stainless steel is widely used as a metal plate for metal masks manufactured by etching or laser processing because it has excellent mechanical strength and corrosion resistance. In recent years, as the performance of laser processing machines has improved, not only has there been an increase in processing accuracy but it has also become possible to cope with short deadlines for orders. As a result, the demand for laser cut metal masks made of stainless steel is increasing.
Technical advances in laser processing machines have result in increases in the processing accuracy of small holes in a laser cut metal mask and the ability to prevent warping due to heat. As a result of these advances, the processing accuracy of laser cut metal masks has been further increasing.
Patent Document 1 discloses that when irradiating a metal plate with a laser beam to form a patterned image having small holes in the metal plate, by suppressing the focal spot diameter of the irradiated laser beam to at most 40 μm and setting the distance between the focal point of the laser beam during processing and the surface of the metal plate on the incident side of the laser beam in the range of −200 to +300 μm, the difference between the diameter of small holes which constitute a patterned image on the incident side of the laser beam and the diameter of the holes on the exit side of the laser beam (referred to in this description as hole spreading) is suppressed to at most 10% of the thickness of the metal plate.
Patent Document 2 discloses a method of manufacturing a laser cut metal mask for screen printing by irradiating a metal plate with a laser beam to melt a portion of the metal plate and bore it to create small holes to form a patterned image and then performing grinding by sandblasting of the surface of the metal plate. When a laser cut metal mask is manufactured by this method, no dross remains, so the rear surface of the mask can contact the printed surface of an object to be printed. In addition, since the surface is textured due to sandblasting, the mask easily releases from the object being printed and the speed of printing can be increased.
Patent Document 3 proposes employing chemical polishing to remove dross or the like which is formed on the rear surface by laser processing.
As shown by Patent Documents 1-3, up to now, the performance of laser cut metal masks has been improved by increasing the processing accuracy of small holes by improvements in laser processing methods such as technical advances in processing tools by carrying out mechanical and/or chemical processing of a metal mask after laser processing in order to stabilize the amount of solder which is supplied through the holes.